Electric induction heating can be used to heat treat tubular materials such as metal tubes and pipes. Typically the tubular material is held in place within a solenoidal induction coil as illustrated in FIG. 1. Tube 90 is placed within solenoidal coil 30. When suitable ac power is applied to the coil, the tube is inductively heated by magnetic coupling with the longitudinal flux field established by the flow of ac current through the coil. The heat treatment may be, for example, annealing, normalizing, stress relieving, coating, drying, hardening or tempering of the end of the tubular material. In other applications induction end heating of tubular products can be used for heating ends prior to metal forming operations. Some applications require uniform heating of a specific length of an end portion of the tubular material.
As shown in FIG. 1, when uniform end heat treatment is desired, the tubular material is situated in the coil so that the coil “overhangs” the end of the material. Generally the longitudinal axis, X, of the coil and tubular material are coincident and the solenoidal coil is shaped to coincide with the shape of the tubular material. The overhang distance, Xoh, controls the shape of the flux field established at an axial end of the coil beyond the end of the tubular material so that the flux field intensity is established within the end of the material to uniformly heat it to the required length. The proper overhang distance is affected by a number of parameters, including the outside diameter of the tubular material, the material's thickness, physical and metallurgical properties, and the frequency of the ac power applied to the coil. Therefore different coils are required for tubular materials of different sizes, or for heat treating the same tubular material to different end lengths. Compare, for example, FIG. 2(a), FIG. 2(b) and FIG. 2(c) wherein the same induction coil 30 and overhang distance, Xoh, is used to induction heat an end of: (1) tubular material 90a having an outside diameter (OD) equal to OD1 and thickness t1; (2) tubular material 90b having an outside diameter OD2, which is smaller than OD1, and thickness t1; and (3) tubular material 90c having an outside diameter OD2 and thickness t2, which is greater than t1, respectively. As illustrated by the graphs in FIG. 2(d), for tubular material 90a in FIG. 2(a), required end heated length 92, thermal transition zone 94 and cold zone 96 all vary. The term “required end heated length” typically refers to a uniform heating temperature distribution over the required end heated length. Since heat cannot be induced in an end length of the material with an abrupt transition to a “no heat” (or cold) end zone, there is an end length with a thermal transition zone 94 wherein the heat decreases gradually towards the cold zone 96 due to a “soaking” effect whereby heat induced in the required end heated length conducts from the required end heated length 92 towards the cold zone 96. Control of both the required end heated length and the length of the thermal transition zone is important in some heat treatment processes. For tubular materials 90b and 90c in FIG. 2(b) and FIG. 2(c), respectively, due to the electromagnetic end effect that exists at the coil end, the materials are not sufficiently heated along the full length of required end heated length 92′ and 92″, respectively. At the end of the tube there is under-heated zone 91. When it is necessary to heat a tubular material with a smaller OD using a coil designed for a larger OD, the end of the tube will be under-heated (zone 91) due to the reduction of heat sources caused by the electromagnetic end effect. If the tubular material is of the same shape, but fabricated from a material having different physical or metallurgical properties, for example a metal that has higher electrical resistivity, then the end of the tube will also be under-heated due to the reduction of heat sources caused by the electromagnetic end effect.
Alternatively a single coil with multiple taps of ac power connections along the length of the coil would allow some additional flexibility for uniform tubular end heating of tubular materials of different dimensions or metallurgical composition. By using appropriate taps for ac power connection, the energized length of the coil can be changed to adjust the overhang distance. Unfortunately, there is a limitation in using coil overhang for obtaining a uniform end heating. This limitation is particularly noticeable when heating magnetic metals below Curie temperature. After reaching certain values, a further increase in coil overhang will not compensate for the lack of heat sources caused by the electromagnetic end effect. In addition, large coil overhangs result in a reduction in coil efficiency and coil power factor. Both factors negatively affect cost effectiveness and flexibility of an induction system due to higher energy losses and the necessity to use special means for coil power factor correction.
One object of the present invention is to improve the end temperature heating uniformity of various types of tubular materials in an electric induction heat treatment process wherein at least one end region of the tubular material is inserted into a solenoidal induction coil. Another object of the present invention is improving flexibility of the induction heating system to permit required (for example, uniform) heating of tubular products of different geometries and materials using the same induction heater.